Because of the lack of knowledge of true causes of low back pain (LBP), a great variety of non-operative treatment methods have been recommended and tried. Recently a new regime, called "back school" was introduced for the purpose of rapidly and economically returning a patient to full functional fitness and normal occupational activities. Assuming that this functional and vocational restoration is a goal of a rehabilitation program, it is apparent that early recovery and higher success rates are major concerns. Hence, it is important to consider these two parameters in evaluating any individual treatment modality or the rehabilitation program as a whole. It is proposed to construct a statistical model (mixture model) that can be used for the investigation of the recovery process of LBP patients in a rehabilitation program. The model provides (i) the percentage of patients who will be recovered, and (ii) the rate of recovery of function. The model will then be used to identify the factors that influence the process of LBP rehabilitation significantly. Approximately 400 LBP patients who participated in the program offered by one of the large urban rehabilitation centers will be studied. In this study "recovery" is defined as a return to work at least on a half time basis.